1. Field of the Invention
This invention relates to a method and apparatus useful in the reclamation of post-consumer plastic films and more particularly to the cleaning and recycling of grocery bags, trash bags, plastic liners and industrial stretch films.
2. Description of Related Art
Plastic recycling has increased significantly in recent years, especially the recycling of plastic containers made, for example, of high density polyethylene (HDPE) or polyethylene terephthalate (PET). HDPE has been increasingly recycled back into rigid containers and into many injection molding applications. Recycling post-consumer plastic materials back into film is especially difficult, however, due to the purity required to produce acceptable quality and performance.
Because of the widely varying nature and quantity of contaminants present in post-consumer plastics presented for recycling, achieving a consistently acceptable quality level in the resultant reclaimed product is a major challenge, particularly when the objective is also to make the reclaimed plastic economically competitive with virgin material. Thin gauge plastic films (e.g., 2 mils thickness or less) are very difficult to decontaminate during reclamation because of their high surface area to volume ratio and their tendency to shroud, envelop, adhere to or otherwise entrap contaminant materials, especially contaminants such as paper. Thorough and efficient removal of these contaminants is desirable, however, and very important if the reclaimed plastic is to be recycled into plastic film applications.
Methods and apparatus for use in recycling plastic films are disclosed, for example, in U.S. Pat. No. 5,268,074 and application Ser. No. 854,231, filed Mar. 20, 1992, both of which are incorporated by reference herein. A separation tank useful for separating plastic from paper and other contaminants has previously been disclosed in U.S. Pat. No. 5,088,910, also incorporated by reference herein.
Notwithstanding the benefits and advantages achieved through use of the methods and apparatus previously disclosed, methods and apparatus are needed that will further improve the efficiency with which post-consumer plastic film is reclaimed, and the quality of the resultant reclaimed plastic. In particular, there is a continuing need for methods and apparatus useful for decontaminating and processing thin gauge plastic film having incoming contamination levels of cellulosic materials and other waste greater than about 4 percent (frequently 8 to 15 percent and sometimes as high as 30 to 40 percent by weight) into a form suitable for use in film applications wherein the recycled film content is as great as about 50 percent recycled material or higher.
Due to the typically high level of contamination in post-consumer plastic films, dry processes cannot be used successfully for recovering the film. Similarly, wet processes that simply utilize "float and sink" methods do not remove contamination sufficiently to allow the material to be reprocessed at significant percentage levels into thin-gauge films of acceptable quality. Even when the post-consumer film reclaimed by conventional means is extruded and melt filtered through a screen-pack, remaining contaminants frequently show up as gels when the reclaimed material is reprocessed into film.
A need therefore exists for an effective and economical method and apparatus for use in cleaning and recovering highly contaminated thin-gauge plastic film suitable for reprocessing into thin-gauge films having a significant recycle content where high levels of film quality and performance are required.
It has also been determined that additional phenomena can be introduced into the film materials by mechanical processing that make cleaning the film and separating contaminants even more difficult. In particular, it has been determined that the shredding and wet grinding steps used to reduce the size of the film pieces can create a static charge that hinders subsequent separation of the film pieces from fine contaminants, particularly fine paper particles that may be present in film from ground or shredded post-consumer trash or grocery bags. Additional methods and apparatus are therefore needed to remove these adherent contaminants from the film particles so that the resultant recovered product will be of acceptable quality for recycling into thin-gauge film.